The present invention relates to an automatic transmission for use with vehicles and, more particularly, to a vehicular automatic transmission for setting a desired speed stage by engaging/releasing a plurality of frictional engagement means such as clutches in accordance with a plurality of kinds of combination patterns for engagement/release of the frictional engagement means.
A vehicular automatic transmission generally uses a plurality of planetary gear sets, each of which is composed of rotary components such as a sun gear, a ring gear and a carrier. The automatic transmission is constructed by connecting predetermined ones of the rotary components, by connecting any of the rotary components selectively to an input shaft through clutch means, by fixing another rotary component selectively by brake means, and by connecting an output shaft to still another rotary component. In the automatic transmission thus constructed, the number of speed stages to be set, the gear ratios at the individual speed stages, and the rotational speed, loads or torques of the rotary components are varied in dependence upon the number of the planetary gear sets used, the manner how to connect the individual rotary components, and the number of and the manner how to dispose frictional engagement means such as the clutch means or the brake means, so that a remarkable variety of automatic transmissions can be constructed. All the structures could not be put into practical use, although they could be made in principle. It is not easy in the least to create a structure which can sufficiently meet the practical requirements such as feasible production, small size and low weight, ample shift controllability and excellent durability. For example, even an automatic transmission using three planetary gear sets to set forward five and reverse one speed stages might have an excessively large gear ratio for the reverse stage in dependence upon the manners how to arrange the frictional engagement means such as the clutches and how to adopt the gear ratio (i.e., the ratio of tooth numbers of the sun gears and the ring gears). For this automatic transmission, it would be necessary to increase the capacities of the frictional engagement means, thus raising a problem that the durabilities of the frictional engagement means and the bearings might be deteriorated. In order to solve this problem, as disclosed in Japanese Patent Laid-Open No. 60-57036, there is proposed an automatic transmission which is constructed to have a reverse gear ratio approximate to that for the forward 1st speed.
The structure of the automatic transmission according to this proposal will be briefly described in the following. This automatic transmission is composed mainly of three single pinion type planetary gear sets and is constructed by connecting the sun gear of the first planetary gear set and the sun gear of the second planetary gear set always or selectively, by connecting the sun gear of the first planetary gear set and the carrier of the second planetary gear set, if necessary, through a clutch, by connecting the ring gear of the second planetary gear set and the ring gear of the third planetary gear set, by connecting the carrier of the first planetary gear set with those ring gears, and by connecting the carrier of the second planetary gear set and the sun gear of the third planetary gear set through a clutch. The input shaft is connected through a clutch to the sun gears of the first and second planetary gear sets, which are connected to each other, and further to the ring gear of the first planetary gear set through another clutch. On the other hand, the output shaft is connected to the carrier of the third planetary gear set. As the brake means for stopping the revolutions, moreover, there are provided a brake for fixing the sun gear of the first planetary gear set and the sun gear of the second planetary gear set and a brake for fixing the sun gear of the third planetary gear set.
On the other hand, the structure, in which a one-way clutch is interposed between the carrier of the second planetary gear set and the casing, is shown in skeleton diagram in the aforementioned Japanese Patent Laid-Open No. 60-57036. However, this Laid-Open has failed to disclose the corresponding clutch and brake application chart and describe the speed stages to be set. Therefore, it is conceivable that the structure arranged with the additional one-way clutch sets the individual speed stages like other embodiments disclosed.
The automatic transmission thus disclosed in the prior art is enabled to set forward seven speeds and reverse one speed as a whole by adding 2.5th and 3.5th speeds to the main speed stages of forward five and reverse one speeds. However, the combination for engagement/release of the clutches and the brakes for setting each of the speed stages is only one. In case, therefore, a shift valve for actuating any clutch or brake is troubled, if the oil passage leading to the shift valve is clogged or if a solenoid valve for operating the shift valve is troubled, that clutch or brake turns inactive so that the speed stage to be effective thereby cannot be set. In case such failure occurs, what can be done by the automatic transmission of the prior art thus far described is to set another in place of the speed stage which cannot be set. In this case, the automatic transmission may have its running performance deteriorated due to shortage of the drive force or reduction of the vehicle speed even with a sufficient drive force being obtained.